1. Field of the Invention
This invention relates to toothed gears for mechanical power transmission and in particular, to rigid toothed gears having resilient coatings affixed to their engaging surfaces.
2. Description of the Prior Art
Gearing conventionally consists of rigid gears having rigid teeth, which transmit mechanical power by forces transmitted from th teeth of one gear to the teeth of a mating gear. As the gears rotate, the point of contact on any tooth moves from tip to root or from root to tip. This contact consists of both a rolling motion and a sliding motion, the sliding motion having a velocity which reverses direction as the point of contact crosses the meshing circle of the gear. The meshing circle is determined by the center distance of the gears and the numbers of teeth, but does not depend upon the geometry of the gears. The sliding motion between the teeth results in friction. The friction in turn causes noise and thermal expansion of the gear teeth. Therefore, to prevent binding of the gear teeth because of thermal expansion, the tooth thickness of at least one gear must be slightly smaller than the space between the teeth of the opposed mating gear.
These clearances between the teeth of mating gears are called "backlash". Backlash is defined as the difference between the space between the teeth of a first gear less the thickness of the teeth of the mating gear. Also contributing to the total backlash are inaccuracies in the machining and formation of the gears.
Because of the clearances between the teeth of the mating gears. The gear teeth engage on only one side of their profiles. Also, in most instances only one pair of teeth are engaged at any particular moment. Therefore, since only a fraction of the gear teeth surfaces are engaged at any one moment, the load capacity of the gearing is substantially reduced.
In order to reduce backlash and noise toothed gears can be made from relatively soft and deformable materials such as rubber. However, when a high load is applied to gear teeth made of rubber, the teeth distort and may distort to such an extent that the teeth of mating gears slip over one another without power transfer.
A solution to this distortion problem is disclosed by Beddoe in U.S. Pat. No. 3,602,058. Beddoe teaches a composite gearing wherein the load is transmitted by deformable teeth when the load is low and by an adjacent set of hard teeth when the load is high. However, this type of system still has the basic problem of friction between the engaging gear teeth as well as increased complexity.
A partial solution to the backlash problem is the use of spring loaded split gears. The springs reduce the effect of backlash by preloading the gears to reduce clearances between the engaging gear teeth. This solution, however, still has the problem of noise and friction.
Methods of reducing friction between engaging gear teeth include the use of lubricants and/or coatings on the gear teeth. Although those methods reduce friction between the engaging teeth, they do not eliminate it.
In many applications the reduction of noise is a critical factor. For example, the primary means of detection of submarines is an underwater acoustic method by which the sounds emitted from the various components of the vessel are detected. The major proportion of this sound is produced by the gearing used in submarine machinery. By eliminating or greatly reducing the intensity of this sound, detection can be avoided.
Prior methods of reducing noise include the use of resilient coatings affixed on the engaging surfaces of gear teeth. These coatings operate to dampen the vibrations caused by the sliding of gear teeth. However, these methods do not eliminate the main cause of noise, theremal expansion and wear, the sliding between engaging teeth.